We present simulations of the detection probability for absorption lines from ions in the warm and hot ionized medium (WHIM) with Athena in the spectra of Gamma-ray burst afterglows. The simulations are based on Swift XRT lightcurves of these afterglows and are performed using the end-to-end simulation framework SIXTE. We simulate both the case of single and multiple absorption lines, as well as results for line searches in absorption structures from a more complex medium. We show that the

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... a X-IFU can detect WHIM lines with strong Ovii lines (equivalent widths larger than 0.14 eV) in spectra containing 3 × 10 6 counts. Downloaded From: http://proceedings.spiedigitallibrary.org/ on 08/17/2016 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx Proc. of SPIE Vol. 9905 99055F-2 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 08/17/2016 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx Proc. of SPIE Vol. 9905 99055F-7 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 08/17/2016 Terms of Use: http://spiedigitallibrary.org/ss/termsofuse.aspx